This invention relates to methods and apparatus for determining the degree of completion and residual lignin content in a digesting or delignification reaction. More particularly, the invention provides an integral part of a closed-loop control system for controlling a digester so as to produce pulp with the proper amount of "cook" for the particular desired paper product.
The sequence of operations in the chemical manufacture of wood pulp preparatory to the production of paper includes debarking of the wood, chipping or fragmenting of the wood, pulping of the chipped wood, and subsequent bleaching, washing, cleaning and screening of the pulp. Although the pulping step is fundamental to the production of paper, the pulping reaction itself is not completely understood. Commonly used chemical pulping processes include the sulfite process and the kraft (or basic) process. While the kraft process is the dominant process practiced industrially, accounting for approximately three-fourths of the total pulp production, fine and writing papers are generally produced using the sulfite process. Both processes are conducted by both batch and continuous methods.
Generally, the control of either pulping process has been accomplished by means of an open loop, feed forward system. According to the most common method, the initial charge conditions such as chip moisture content, sulfidity, liquor to wood ratio, and acidity or alkalinity are monitored. Cooking over a given time and at a given temperature for particular values of these conditions is expected to result in the same pulp yield and Kappa number, or lignin content. The cook is then stopped at a time which is expected to give an acceptable pulp yield. Hence this conventional method is used to predict when to stop the cook in order to achieve a desired pulp yield. Upon stopping the cook, the pulp is analyzed to ascertain whether the desired degree of completion has in fact been attained. The results of this analysis are used to adjust the cooking parameters to be used on subsequent cooks.
One clear disadvantage of this conventional method is that control of the product of the pulping operation is dependent upon the maintenance of uniform conditions of chip moisture content, temperature and chemical properties. It is very difficult to accurately ascertain certain of these variables such as chip moisture in any practical way. Accordingly the results of one reaction are used to correct the conditions for the next run in a trial and error operation based on the questionable assumption that the average properties of the components remain substantially uniform from one cook to the next. Thus it has not been possible to close the open control loop of the conventional method because of the lack of suitable process measurements.
Another approach to controlling the completeness of the pulp delignification process has been the method of measuring the "Kappa number", which is defined by the Technical Association of the Pulp and Paper Industry (TAPPI) standard T-236. The known relationship between the Kappa number and the lignin content is as follows: the percent lignin in the pulp equals 0.147 times the Kappa number. The Kappa number method, however, is not suitable for on-line measurement because the pulping reaction must be terminated and the pulp oven-dried in order to measure the Kappa number. Certain models have been developed over time to aid in inferring the Kappa number from measurement of other parameters, such as the measurement of sulfidity of the liquor by a selective ion electrode, or the titration of a liquor sample by acid. These methods have not received wide used within the industry. Virtually all of the pulping control done in this country employs the conventional method described above or equivalent feed forward control.
The patent issued to Pottenger, U.S. Pat. No. 3,674,434, teaches a system for determining the lignin content "directly", by measuring the ratio of elemental carbon to elemental hydrogen in a sample. Since this method requires termination of the reaction for direct sampling of the pulp, it is unsuited for online real time monitoring and process control.
The patent issued to Andrews, Jr., et al., U.S. Pat. No. 4,193,840, teaches a method for determining the degree of delignification by monitoring a combination of temperature and pressure. Because these parameters are only indirectly related to the lignin content of the pulp, reliability of the measurements is suspect.
The patent issued to Chase, et al., U.S. Pat. No. 4,162,933, determines the degree of delignification by monitoring heat given off by the chemical reaction of the digesting operation. Once again, the information taken is only indirectly related to the lignin content of the pulp, and so the reliability required for an on-line process control system is unavailable.
The Institute of Paper Chemistry (IPC) has disclosed a technique using the carbohydrate fraction of pulp to predict yield in continuous digesters. The method assumes that the yield of cellulose as a percentage of a particular wood species is substantially constant during a Kraft reaction. The method requires extremely accurate determination of pulp carbohydrates, such as by gas chromatography, thus being too expensive to be widely applicable to on-line real time measurement of pulp yield.
This invention relates to improvements over the methods and apparatus described above and to solutions to the problems raised thereby.